A display device utilizing a liquid crystal compound (in this patent application, a liquid crystal compound is used as a generic term for a compound that exhibits a liquid crystal phase and a compound that exhibits no liquid crystal phases but useful as a component of a liquid crystal composition) has been widely used for the display of a watch, a calculator, a word processor or the like. The display device utilizes the refractive index anisotropy (Δn), the dielectric anisotropy (Δε) and so forth of the liquid crystal compound.
A liquid crystal phase includes a nematic liquid crystal phase, a smectic liquid crystal phase and a cholesteric liquid crystal phase, and the nematic liquid crystal phase is most widely applied. A display mode includes a DS (dynamic scattering) mode, a DAP (deformation of aligned phases) mode, a GH (guest-host) mode, a TN (twisted nematic) mode, a STN (super twisted nematic) mode, a TFT (thin film transistor) mode, a VA (vertical alignment) mode, an IPS (in-plane switching) mode and a PSA (polymer sustained alignment) mode.
A liquid crystal compound used for these display modes is required to exhibit a liquid crystal phase in a wide temperature range, centering at room temperature, to be sufficiently stable under conditions in which the display device is used, and also to have sufficient characteristics for driving the display device. However, no single liquid crystal compound that satisfies these conditions has been found until now.
The actual situation is that a liquid crystal composition is prepared by mixing from several to several tens of liquid crystal compounds in order to satisfy the required characteristics. It is required that the liquid crystal composition is stable to moisture, light, heat and air, which are normally present under conditions in which the display device is used, and is stable to an electric field or electromagnetic radiation, and is also stable chemically to a compound that will be mixed. It is required that the liquid crystal composition has suitable values of a variety of physical properties such as refractive index anisotropy (Δn) and dielectric anisotropy (Δε), depending on the display mode or the shape of the display device. Furthermore, it is important that each component in the liquid crystal composition has an excellent solubility in each other.
In recent years, modes such as IPS, VA, OCB and PSA among the display modes have been receiving attention as a display mode capable of overcoming a narrow viewing angle of a liquid crystal display device, which is the greatest subject to be solved. In liquid crystal display devices having these modes, a liquid crystal display device, especially having the VA mode or the IPS mode, has been studied earnestly, since it has an excellent responsivity in addition to a wide viewing angle, and is capable of providing a high-contrast display. The liquid crystal composition used in the liquid crystal display devices having these display modes is characterized by the negative dielectric anisotropy (Δε). It is known that a liquid crystal composition having a large negative dielectric anisotropy (Δε) can decrease the driving voltage of a liquid crystal display device containing the liquid crystal composition (Non-patent document No. 1). Accordingly, liquid crystal compounds as the components of the liquid crystal composition are also required to have a larger negative dielectric anisotropy (Δε).
A variety of liquid crystal compounds in which lateral hydrogen on the benzene ring had been replaced by fluorine have conventionally been studied as a component of a liquid crystal composition having negative dielectric anisotropy (Patent documents No. 1 and No. 2). For example, a compound represented by formula (a) has been reported (hereinafter, it may be abbreviated to the compound (a); the same applies to a compound represented by another formula, giving a term such as compound (1-1)). However, the compound (a) did not always have a large value although it had negative dielectric anisotropy (Δε) and the value might not be sufficiently large for decreasing the driving voltage of a liquid crystal display device having a VA mode, an IPS mode or the like.
In formula (a), R and R′ are alkyl.
Thus, in a compound having a 2,3-difluorophenylene moiety, attempts have been made to increase the absolute value of negative dielectric anisotropy (Δε). For example, a compound having the 2,3-difluorophenylene moiety, to which a tetrahydropyran-2,5-diyl moiety is introduced has been reported (Patent document No. 3). The compound (b) has a large negative dielectric anisotropy (Δε) in comparison with the compound (a).

On the other hand, a liquid crystal composition having a wide temperature range of a nematic phase has been requested along with an increased use of a liquid crystal display device, and thus a liquid crystal compound having a high clearing point has been requested. There is generally a tendency to increase the viscosity of a liquid crystal compound as the clearing point increases. However, a low viscosity is desirable in order to achieve a high response speed in the liquid crystal composition. Because of this point of view, a liquid crystal compound having a large negative dielectric anisotropy (As) and having both a high clearing point and a low viscosity has been expected.